Do you know the person you are trying to hack personally? If So, ask them questions about their life. Questions that would be used for "Security" questions on gmail. (or facebook) Do a bit of research and make a fake account, but when It comes to the part about security questions, browse through the lists of them, and writ down each one of them. Then, seemingly nonchalantly, ask them those questions. (you will need to paraphrase, of course, to seem non-intrusive) And then, when using their Email to get into their FB, or Gmail, or whatever, enter the answers to the security questions, and boom, account hacked.

(sorry if I babbled lol. I just love Social Engineering. Humans are so easily manipulated.)

EDIT: If you're going for the technical and far better way for social engineering, definitely use the link Cracker|Hacker gave you. I'm just one of the people who like manipulating their "victims" In person. Also, you really shouldn't be doing this in the first place if you're going to cause mischief.

Yes, but you must have a linux penetration distro to do it (I recommend Kali Linux) Cracker|Hacker has already linked to you "Hydra". Follow the tutorial to use that, and you should be good to go. (assuming you get it right in the first 500 tries which is actually quite hard from my experiences.)